The present invention relates to a power servo booster particularly one of the fluid differential pressure actuated type having two power pistons connected in series. Even more particularly, the invention relates to such a power servo booster in which the diaphragms thereof are prevented from being folded to provide surface contact in a return stroke of a power piston during the non-operative state of the booster.
A conventional power servo booster is shown in FIGS. 1 through 3 in which a power piston 1 is slidingly disposed within a shell 5 and a diaphragm 2 is secured to divide an internal space of the shell 5 into a negative pressure chamber 3 and an atmospheric pressure chamber 4. In the non-operative state of the booster in which a negative pressure is introduced to the negative pressure chamber 3 through a connecting tube 6, negative pressure is also introduced into the atmospheric pressure chamber 4 from the negative pressure chamber 3 through a valve device (not shown) disposed in the power piston 1. The position of the power piston is maintained as shown by a return spring 7. Upon urging or axial displacement of an input shaft 8 so as to cause fluid communication between the negative pressure chamber 3 and the atmospheric pressure chamber 4 to be blocked, atmospheric pressure is introduced into the chamber 4 to provide a pressure differential between the chambers 3 and 4 to thus create a force on the power piston 1 which in turn displaces a push rod 9 along the axial direction thereof.
When no negative pressure is present in the negative pressure chamber 3 such as during periods of air discharge in the brake lines, the internal pressure of the negative pressure chamber 3 becomes relatively larger than that of the atmospheric pressure chamber even if these chambers were both in communication with the atmosphere because of changes in relative volumes in the two chambers due to movement of the input shaft 8. As a result, a disc portion 2a of the diaphragm 2 is urged rearwardly in spite of the forward movement of the power piston 1 so that the diaphragm 2 becomes spaced apart from the power piston 1 and is urged toward its original non-powered position. In this case, as shown in FIG. 2, a peripheral end of a disc portion 1a of the power piston 1 extends beyond a folded portion 2b of the diaphragm 2 and the folded portion 2b is inclined radially inwardly as shown by an arrow. When the power piston 1 is restored to its original position, with such a deformation of the diaphragm maintained, portions of the diaphragm 2 may be disadvantageously folded over one another as shown in FIG. 3.
Such diaphragm folding or collapsing is most apt to occur in a power servo booster in which volume changes of the chambers are relatively large with respect to the stroke length of the power piston, particularly in a tandem type power servo booster provided with two negative pressure chambers in fluid communication with each other and two atmospheric chambers in fluid communication with each other.
The inventor of the present application has found that such collapsing of the folded diaphragm portion 2b in the direction shown by the arrow in FIG. 2 causes excessive bending at the boundary portion A of the diaphragm between an inner peripheral end portion 2c and the disc portion 2a thereof. That is, in a conventional power-servo booster, the inner peripheral end 2c of the diaphragm is fixedly secured to an annular groove 1b formed in the power piston 1 so that the inner peripheral end 2c is not freely rotatable or pivotable or movable in any manner. Even if a minute clearance were to be provided between the inner peripheral end 2c and the annular groove 1b with the end 2c more easily engaged with the groove 1b, such a clearance would nontheless not allow the end 2c to be freely movable in the groove 1b. Therefore, the peripheral end of the disc portion 1a of the power piston 1 would still tend to extend beyond the folded portion 2b of the diaphragm so that the boundary portion A of the diaphragm would still tend to bend and be deformed as shown in FIG. 2 with the folded portion 2b urged radially inwardly due to the application of stress thereto.